1. Field of the Invention
The present invention relates to a two-wheel/four-wheel change-over device in an automobile transfer apparatus wherein the change-over device usually serves as a direct connection type transfer apparatus and is capable of changing over to two-wheel drive upon demand.
2. Description of the Prior Art
A conventionally proposed control unit in an automobile transfer apparatus disclosed in the specification of Japanese Utility Model Publication No. 55055/1981 involves a mode of operation (an easy drive mode) in which a central differential is provided. Driving forces can be imparted to individual wheels in proportion to a distribution ratio even when the four wheels respectively have a difference of rotation (no undesirable influence is exerted on the rotation of the wheels).
The driving forces are, however, determined by a coefficient of friction between tires, road surface and the car weight that is to be applied to the tires, so that a slip-state could be created even in a single wheel. As a result, other wheels come to have the driving forces corresponding to a degree to which the gripping power is reduced. Hence, it is required to prepare a lock mechanism for avoiding such a situation. Addition of this lock mechanism, however, makes a structure of the transfer apparatus more complicated.
In another transfer apparatus wherein two-wheel/four-wheel drive, which is referred to as a part-time mode, is manually changed over when four-wheel drive travelling is carried out, the driving forces of all the wheels are, as described above, by no means solely determined by the gripping power of one wheel. However, when the wheels rotate, the rear wheels travel inside the front wheels, i.e., understeer. Since no difference of rotation exists between a front propeller shaft and a rear propeller shaft, there is produced a so-called stick slipping phenomenon, i.e., a tight corner braking phenomenon wherein the difference of rotation escapes, such a phenomenon existing between the road surface and the tires. This causes defects such as abrasion of the tires, increases cost of fuel, and the riding comfort is decreased by vibration noises.
In order to solve the above-described problems, as is disclosed in the specification of Japanese Utility Model Publication No. 26636/1983, a full-time type four-wheel driving device is proposed. This device is arranged such that a clutch engaging state which ordinarily permits a slide by forces provided by springs, and when the rotational difference, i.e., slip between the front and rear wheel is created, completed integration is obtained with the aid of a hydraulic piston. In examining travelling conditions of the automobile, however, there is a difference in the amount of air in the tires of the front and rear wheels, and further a difference in load-radius between the tires which is caused by loading of the vehicle. Therefore, the clutch member may be fixedly engaged, this leading to problems of friction loss of the clutch and an increase in heat.
Under ordinary conditions of travelling (on dry and flat surface of a concrete road), the two-wheel travelling is sufficient and four-wheel travelling is not required. Besides, the four-wheel travelling has disadvantages pertaining to noise and the cost of fuel. An additional problem incidental to four-wheel travelling is that when diminishing a time-lag between increasing a predetermined pressure and attaining the complete engagement of the clutch by operating the hydraulic piston after detecting a slip-rate, a hydraulic pump having a large capacity has to be utilized and the power needed to drive the pump results in an increase in the cost of fuel.